Remains of fossil organisms, including plants, insects (beetles, chironomids), ostracods and gastropods, preserved in frozen deposits of a shallow paleo-lake indicate boreal climate at todays Arctic mainland coast of Oyogos Yar, Yakutia during the last interglacial about 125.000 years BP. Terrestrial beetle and plant remains prove the former existence of open forest tundra with Dahurian larch (Larix dahurica), grey alder (Alnus incana) and boreal shrubs (Duschekia fruticosa, Betula fruticosa, B. divaricata, B. nana) interspersed with patches of steppes and meadows. Consequently, the tree line was shifted northward up to todays mainland coast, and probably beyond, thus at least 270 km off its current position. Low net precipitation is shown by steppe plants and beetles. Aquatic organisms such as chironomids, ostracods, gastropods and hydrophytes indicate lake formation as result of thermokarst processes. Many of the lacustrine organisms are very warmth sensitive indicating a mean temperature of the warmest month near 13 °C, which is above the minimum requirement of tree growth and in contrast to modern values of less than 4 °C (Voeikov Main Geophysical Observatory, 1980). Littoral pioneer plants and chironomids indicate intense lake level fluctuations due to high evaporation. The results suggest that modern Arctic tundra, lacking woody plants and bound to constantly wet ground under cool and moist summer conditions, did obviously not exist in Arctic Siberia in warm stages prior to the Holocene.